1. Field of the Invention
The present invention relates to an apparatus and method for washing lump or rod-shaped polycrystalline silicon, which is a raw material of single crystal silicon for semiconductors and silicon for solar cells.
Priority is claimed on Japanese Patent Application No. 2008-303822, filed Nov. 28, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
In general, single crystal silicon used for semiconductor devices is produced by a Czochralski method. In the Czochralski method, lump or rod-shaped polycrystalline silicon or single crystal silicon is melted in a quartz crucible, a seed crystal is dipped into the obtained melt, and the seed crystal is pulled to grow a single crystal silicon. In order to obtain high-quality single crystal silicon, it is necessary to minimize the amount of impurities adhered to the surface of the lump or rod-shaped polycrystalline silicon.
In order to minimize the amount of impurities, a method has been used which washes the polycrystalline silicon produced in a predetermined shape with chemicals to remove the impurities adhered to the surface of the polycrystalline silicon.
As the chemicals for removing the impurities, for example, the following have been used: a mixture of hydrofluoric acid, hydrogen peroxide aqueous solution and water (see Japanese Unexamined Patent Application, First Publication No. H5-4811); pure water (see Japanese Unexamined Patent Application, First Publication No. 2002-293688 and Japanese Unexamined Patent Application, First Publication No. 2007-313454); and a mixed solution of a nitric acid and a hydrofluoric acid (see Japanese Unexamined Patent Application, First Publication No 117-187900). The surface of the polycrystalline silicon is washed using these chemicals.
In the method disclosed in Japanese Unexamined Patent Application, First Publication No. H5-4811, since an etching reaction is weaker than that in the method disclosed in Japanese Unexamined Patent Application, First Publication No. H7-187900, impurities are likely to remain on the surface of the polycrystalline silicon. In the methods disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-293688 and Japanese Unexamined Patent Application, First Publication No. 2007-313454, washing is performed with pure water and there is no etching reaction. Therefore, it is difficult to remove the impurities strongly adhered to the surface of the polycrystalline silicon. That is, the quality of the surface of the washed polycrystalline silicon strongly depends on the state (quality) of the surface of the polycrystalline silicon before it is put into a washing apparatus.
In the method disclosed in Japanese Unexamined Patent Application, First Publication No. H7-187900, the surface of the polycrystalline silicon is washed by an etching reaction. The processing stages are continuously performed to prevent the generation of an oxide film, which is viewed as spots (hereinafter, referred to as a spot), which causes a deterioration in the quality of the polycrystalline silicon. In addition, an etchant is appropriately supplied to maintain a uniform etching reaction. However, since the polycrystalline silicon and the etchant can be exchanged between the baths, there is a concern that the impurities dissolved in a liquid during etching reaction in the first half of a process will be diffused into the etchant used in the second half of the process. As a result, the impurities are also adhered to the surface of the polycrystalline silicon, which makes it difficult to obtain high-quality polycrystalline silicon.